1. Technical Field
The present disclosure relates to microwave generators and, more particularly, to systems and methods for using a digital controller to adjust one or more operations of a microwave generator.
2. Discussion of Related Art
Electromagnetic fields can be used to heat and destroy tumor cells. Treatment may involve inserting ablation probes into tissues where cancerous tumors have been identified. Once the ablation probes are properly positioned, the ablation probes induce electromagnetic fields within the tissue surrounding the ablation probes.
In the treatment of diseases such as cancer, certain types of tumor cells have been found to denature at elevated temperatures that are slightly lower than temperatures normally injurious to healthy cells. Known treatment methods, such as hyperthermia therapy, heat diseased cells to temperatures above 41° C. while maintaining adjacent healthy cells below the temperature at which irreversible cell destruction occurs. These methods involve applying electromagnetic fields to heat or ablate tissue.
Devices utilizing electromagnetic fields have been developed for a variety of uses and applications. Typically, apparatuses for use in ablation procedures include a power generation source, e.g., a microwave generator that functions as an energy source, and a surgical instrument (e.g., microwave ablation probe having an antenna assembly) for directing energy to the target tissue. The generator and surgical instrument are typically operatively coupled by a cable assembly having a plurality of conductors for transmitting energy from the generator to the instrument, and for communicating control, feedback, and identification signals between the instrument and the generator.
There are several types of microwave probes and waveguides in use, e.g., monopole, dipole, and helical, which may be used in tissue ablation applications. In monopole and dipole antenna assemblies, microwave energy generally radiates perpendicularly away from the axis of the conductor. Monopole antenna assemblies typically include a single, elongated conductor. A typical dipole antenna assembly includes two elongated conductors that are linearly-aligned and positioned end-to-end relative to one another with an electrical insulator placed therebetween. Helical antenna assemblies include helically-shaped conductor configurations of various dimensions, e.g., diameter and length. The main modes of operation of a helical antenna assembly are normal mode (broadside), in which the field radiated by the helix is maximum in a perpendicular plane to the helix axis, and axial mode (end fire), in which maximum radiation is along the helix axis.
The heating of tissue for thermal ablation is accomplished through a variety of approaches, including conduction of heat from an applied surface or element, ionic agitation by electrical current flowing from an electrode to a ground pad, optical wavelength absorption, or, in the case of microwave ablation, by dielectric relaxation of water molecules within an applied electromagnetic field. Regardless of the approach, conceptually thermally ablative devices coagulate and necrose tissue with two distinct heating zones; an active heating zone and a passive heating zone.
The active ablation zone is closest to the ablation device and encompasses the volume of tissue which is subjected to energy absorption high enough to assure thermal tissue destruction at a given application time in all but areas of very rapidly flowing fluids, such as around and within large blood vessels or airways. The active ablation zone size and shape is determined by ablation device design. The active ablation zone can therefore be used to produce predictable ablative effects over a given shape and volume of tissue.
The passive ablation zone surrounds the active zone and encompasses the volume of tissue which experiences a lower intensity of energy absorption. The tissue within the passive ablation zone may or may not experience tissue destruction at a given application time. Physiological cooling may counter heating from the lower level energy absorption and therefore not allow for sufficient heating to occur within the passive zone to kill tissue. Diseased or poorly perfused tissue within the passive zone may be more prone to heating than other tissues and may also be more susceptible to heat conduction from hotter areas within the ablation zone. The passive zone in these cases can result in unexpectedly large ablation zones. Due to these varying scenarios across space within a targeted physiology, relying on the passive zone to perform thermal ablation is challenging with unpredictable outcomes.
As electromagnetic fields can be induced at a distance by microwave probes, microwave ablation has the potential to create large active zones whose shapes can be determined and held constant by design. Furthermore, the shape and size can be determined through design to fit a specific medical application. By utilizing a predetermined active zone to create a predictable ablation zone, and not relying upon the indeterminate passive ablation zone, microwave ablation can provide a level of predictability and procedural relevance not possible with other ablative techniques.
The shape of the active zone about an antenna is determined by the frequency of operation, the geometry of the antenna, the materials of the antenna, and the medium surrounding the antenna. Operating an antenna in a medium of dynamically changing electrical properties, such as heating tissue, results in a changing shape of the electromagnetic field, and therefore a changing shape of the active zone. To maintain the shape of the active zone about a microwave antenna, the degree of influence on the electromagnetic field of the surrounding medium's electrical properties is reduced.
The size of the active zone about an antenna is determined by the amount of energy which can be delivered from the microwave generator to the antenna. With more energy delivered to the antenna, larger active zones can be generated. To maximize energy transfer from a microwave generator through waveguides and to a microwave antenna requires each system component to have the same impedance, or to be impedance matched. Whereas the impedance of the generator and waveguides are typically fixed, the impedance of a microwave antenna is determined by the frequency of operation, the geometry of the antenna, the materials of the antenna, and the medium surrounding the antenna. Operating an antenna in a medium of dynamically changing electrical properties, such as within heating tissue, results in a changing antenna impedance and varied energy delivery to the antenna, and, as a result, a changing size of the active zone. To maintain the size of the active zone about a microwave antenna, the degree of influence on the antenna impedance of the surrounding medium's electrical properties must be reduced.
In microwave ablation, the primary cause of active zone size and shape change is an elongation of the electromagnetic wave. Wavelength elongation occurs in heating tissue due to tissue dehydration. Dehydration reduces the dielectric constant of tissue about the probe, elongating the wavelength of microwave fields. Wavelength elongation is also encountered when a microwave device is used across various tissue types due to the varying dielectric constant between tissue types. For example, an electromagnetic wave is significantly longer in lung tissue than in liver tissue.
Wavelength elongation compromises the focus of microwave energy on the targeted tissue. With large volume ablation, a generally spherical active zone is preferable to focus the energy on generally spherical tissue targets. Wavelength elongation causes the electromagnetic field to stretch down along the length of the device toward the generator, resulting in a generally comet- or “hot-dog”-shaped active zone.
Wavelength elongation can be significantly reduced in medical microwave antennas by dielectrically buffering the antenna geometry with a material having an unchanging dielectric constant, as described in U.S. application Ser. Nos. 13/835,283 and 13/836,519, the disclosure of each of which are incorporated by reference herein. The material of unchanging dielectric constant surrounds the antenna, reducing the influence of the tissue electrical properties on antenna wavelength. By controlling wavelength elongation through dielectric buffering, the antenna impedance match and field shape can be maintained, enabling a large active ablation zone with a predetermined and robust shape.
By providing dielectric buffering with a circulated fluid, such as with saline or water, the high dielectric constants of these materials can be leveraged in the antenna geometry design, and furthermore the circulated fluid can be used to simultaneously cool the microwave components, including the coaxial feed line and antenna. Cooling of the microwave components also enables higher power handling of the components which can be used to deliver more energy to the antenna to create larger active zones.
Some microwave generators currently on the market have been developed and refined so that no digital circuitry is required for control. Instead, all functions of the device are controlled by analog control systems. This includes the reflective power measurements, the amplifier control circuitry and others. Indeed, in some instances the only digital aspects of these microwave generators or related to the digital display of the timer and or the power setting.
However, microwave surgical instruments are constantly upgraded with new functions that may not be compatible with these existing electrosurgical generators. However, reprogramming or upgrading an electrosurgical generator for the purpose of interacting with new microwave surgical instruments is cumbersome and has its own drawbacks. According there is a need for a system and method of adding or altering or adjusting operations or functionality of existing microwave generators.